There is substantial need for switching power converters to convert high voltage to low voltage. For example, power distribution busses in enterprise-grade information technology equipment commonly operate at a relatively high voltage, such as 48 volts or 54 volts, to help minimize magnitude of current carried by these busses. Many loads in information technology equipment, however, operate at a low voltage. For instance, modern microprocessors often include a processor core that operates at a voltage of around one volt, and modern electronic memory devices typically operate at a voltage of less than two volts. Consequently, switching power converters are required in information technology equipment to convert high voltage to low voltage.
As another example, automotive power distribution busses commonly operate at a nominal voltage of around 14 volts during automobile operation, and switching of automobile electrical loads may cause power distribution buss voltage to significantly exceed 14 volts for short time periods. Many automobile electrical loads, however, require a much smaller voltage, such 3.3 volts. Consequently, switching power converters are also required in automobiles to convert high voltage to low voltage.
A two-level buck converter is capable of converting high voltage to low voltage. However, switching transistors in a two-level buck converter must have a high voltage rating if the two-level buck converter is to be used in high voltage applications. A transistor having a high voltage rating typically has a higher on-resistance for a given area than an otherwise identical transistor having a low voltage rating. Such high on-resistance causes significant resistive power loss at high current levels. Consequently, a two-level buck converter designed for high voltage operation will typically be less efficient at high current levels than an otherwise identical two-level buck converter designed for low-voltage operation.
A multi-level buck converter can be used in place of a two-level buck converter to reduce voltage across transistors, where in this document, the term “multi-level” means three or more levels. For example, FIG. 1 illustrates a conventional four-level buck switching power converter 100 based on the Meynard topology which includes three upper transistors 102, three lower transistors 104, two flying capacitors 106, an inductor 108, an output capacitor 110, and a controller 112. In this document, specific instances of an item may be referred to by use of a numeral in parentheses (e.g., upper transistor 102(1)) while numerals without parentheses refer to any such item (e.g., upper transistors 102). Upper transistors 102 are electrically coupled in series between a power node 114 and a switching node 116, and lower transistors 104 are electrically coupled in series between switching node 116 and a reference node 118. Flying capacitor 106(1) is electrically coupled between a first upper node 122(1) and a second lower node 124(2), and flying capacitor 106(2) is electrically coupled between a second upper node 122(2) and a first lower node 124(1). Inductor 108 is electrically coupled between switching node 116 and a power node 120. An input electric power source 126 having a voltage Vin is electrically coupled between power node 114 and reference node 118, and a load 128 is electrically coupled between power node 120 and reference node 118.
Controller 112 is configured to control switching of upper transistors 102 such that the upper transistors switch out of phase with each other and with a duty cycle that achieves a desired magnitude of an output voltage Vout. Controller 112 is also configured to control switching of lower transistors 104 such that each lower transistor 104 switches in a complementary manner with a respective upper transistor 102. It can be determined that maximum voltage across each upper transistor 102 and that maximum voltage across each lower transistor 104 is equal to Vin/3. Accordingly, the four-level topology of buck converter 100 enables each transistor 102 and 104 to have a significantly lower voltage rating (i.e., Vin/3) than the magnitude of input voltage Vin.